1. Field of the Invention
The present invention is directed to an electrical insulating covering composition for an electrical cable. More particularly, the present invention is directed to a method for covering electrical conducting means wherein said means are covered with a crosslinked ethylene polymer composition.
2. Background of the Prior Art
The use of polymeric compositions as insulating materials for electrical conducting wires and cables is well known. Those skilled in the art are aware that ethylene polymers, especially polyethylene (PE), are particularly preferred in this application. Thus, the art is replete with patents directed to polyethylene cable coatings, wraps and the like. Many PE compositions have been devised to provide various desirable properties particularly designed for this application.
Only recently has there been a significant increase in the trend toward disposing electrical wires, cables and the like underground. Underground wiring eliminates service interruptions in inclement weather caused by felled trees, debris and the like which severs overhead power lines.
Although underground cabling has obvious advantages, it places more severe service requirements upon cable coverings. As those skilled in the art are aware, cables disposed underground are subject to serious abrasion in installation. That is, cables are oftentimes disposed underground in cement or concrete conduits. The coated wires are moved along these conduits, subjecting the cable coatings to abrasion. As a result, cable coatings must not only provide the properties usually associated with coatings for electrically conducting conduits, that is, electrical resistance, flame resistance, resistance to stress cracking and the like but, in addition, abrasion resistance.
Those skilled in the art are aware that for a given molecular weight the abrasion resistance of ethylene polymers is generally proportional to their density. Conversely, at a given density, abrasion resistance of an ethylene polymer is proportional to molecular weight. Thus, the more dense an ethylene polymer the greater the degree of its resistance to abrasion. However, because of the higher melting point of high density ethylene polymers, particularly polyethylene, the more difficult it is to effectuate crosslinkage thereof. This is because the usual crosslinking agents decompose at relatively low temperatures. Unfortunately, the higher the density the higher the temperature required to blend the ethylene polymer with crosslinking agents. Thus, in the past it was not possible to utilize a sufficiently high density ethylene polymer, satisfactory to provide adequate abrasion resistance because such an ethylene polymer could not be crosslinked.
Those skilled in the art are aware that many of the properties, other than abrasion resistance, of an ethylene polymer employed as a cable wrap require that the polymer be crosslinked. Thus, a dilemma in the art arises when an attempt is made to provide a polyethylene or other ethylene polymer cable covering with acceptable abrasion resistance while maintaining other acceptable properties.
U.S. Pat. No. 2,528,523 discloses a process for crosslinking polyethylene to produce a polymer particularly applicable to wire coating. In this process a tertiary peroxide is blended with polyethylene at a temperature of 110.degree. to 180.degree. C. It is noted that the patentee excludes those tertiary peroxides which possess unsaturated linkages or which contain aromatic radicals.
U.S. Pat. No. 3,649,441 sets forth an electrical conductor jacketing material submitted to possess improved thermal and environmental stress resistance as well as abrasion resistance. The material providing these improved properties is a blend of high density polyethylene and a block copolymer of polyethylene and poly(butene-1). Neither of these two blended polymers are crosslinked.